Researchers have found traces of a heat-loving bacterium that may live beneath a frozen lake in Antarctica.

Lake Vostok is covered by more than 3km of ice and must have been isolated from our planet's atmosphere for millions of years.

The bacteria appeared in sediment mixed with a core of ice drilled by Russian and French researchers.

The heat-loving, or thermophilic, bacterium may suggest that hydrothermal vents exist on the lake floor.

Meanwhile, a new ice core drilled this season may reveal whether there is also life in the lake itself.

Hundreds of lakes exist beneath the thick Antarctic ice sheet, but with an area of 14,000 sq km Vostok is by far the largest.

It has never been penetrated, but scientists know it is there through radar measurements taken from above.

Lost world

Because Vostok sees no sunlight and has not been in contact with the atmosphere since it was covered with ice around 15 million years ago, scientists hope it might reveal the kind of life that could exist on other planets, or on Europa, the ice-covered moon of Jupiter.

LAKE VOSTOK - ANTARCTICA

There are more than 70 sub-glacial lakes in Antarctica; Vostok is the biggest

At 14,000 sq km, it is about the extent of Lake Ontario and is up to 500m deep in places.

Its waters have been hermetically sealed from air and light for 15 million years

Overlying ice layers reveal a 400,000-year environmental record with microbes present throughout the core

Many scientists consider Vostok to be a good model for the ecosystems that might exist on Jupiter's frozen moons

All information about the contents of Lake Vostok comes from an ice core drilled by Russian and French scientists.

The core was originally intended to study past climate, but although drilling stopped more than 100m above the lake's surface, the bottom parts of the core turned out to be frozen chunks of the lake water itself.

Now, a team of Russian and French scientists have used this core to investigate what the lake is truly like.

So far they have found no definitive traces of life within Lake Vostok. But the ice also contained streaks of dark sediment that they believe was thrust up from the lake floor by a small earthquake.

In the sediment, the team found genetic traces of a bacterium that usually lives in temperatures of 50-60C.

"We expected to find life adapted to a cold environment but instead we found exactly the opposite," said Jean-Robert Petit of the Laboratory of Glaciology and Geophysics of the Environment in Grenoble, France.

Rare bug

To be sure the bacteria really exist below Lake Vostok, the researchers had to be very careful not to contaminate their sample.

They compiled a list of microbes that could be found in their laboratory, and eliminated every one from their results.

But the heat-loving bacterium appeared only in the ice core. In fact it is quite unusual, making the researchers even more confident that this one must have come from below the lake.

"We were happy because this bacterium has only been found until now in five or six places on Earth," said Sergey Bulat of the Petersburg Nuclear Physics Institute.

They now believe that Lake Vostok may have some kind of hydrothermal heating system at its base, making it more active than anyone had realised.

Concentrated bleach

Meanwhile they are still searching for other evidence of life in the lake water itself.

Recently, American researchers announced they had found microbes that must have come from Vostok lake water, but the results were controversial with others saying they were simply contamination.

The Russian and French team has even suggested that the lake water itself may be sterile. The researchers say that oxygen has been pumped into the lake from the ice above for millions of years. This would have turned the water into a concentrated solution of bleach.

However, if this is true it would be the first truly lifeless body of water ever found on Earth.

New drilling from the Russians may help to resolve this controversy. This season they returned to drill again at Vostok for the first time in eight years.

By extending the Vostok drill hole to a depth of 3,650m, a new world record, they managed to obtain an extra 27m of ice core. Since this ice comes from even closer to the water surface, it should reveal much more about the contents of this mysterious lake.

A report on Lake Vostok was broadcast on the Leading Edge programme on BBC Radio 4. It can be heard again on the Leading Edge website.